"Our boat's about 6m, and the seal was probably three quarters of the length of the boat. It was a big animal, that's for sure."
Penfold said the seal slid off the pontoon and swam towards them, and he "wondered whether it would do anything or not".
His 14-year-old daughter Tayla had her phone ready to snap pictures of their close encounter, and was "very excited".
The North Shore resident added that his wife had joked it would be great if he could organise a whale or a pod of dolphins on the boat ride home, but they weren't expecting a seal.
"We've been up in the Galapagos Islands before, but this is bigger than any sea life we saw there."
Nigel Williams, a marine engineer, also had a close call with the leopard seal in early November.
Williams said he saw the animal twice on consecutive days at the mooring off Hobsonville Point.
"I saw it coming up to the mooring and I thought it was a dog. I had a second look and thought: 'That's a big dog'."
The 57-year-old said it was swimming "rather fast" and making a beeline towards something.
The following afternoon he was rowing his 2.4m dinghy to his yacht, his dog perched on the back, when the leopard seal appeared again.
"I was about halfway from the Hobsonville ramp to the boat. I wondered whether the sea leopard was still around."
Williams said the seal "popped up out of nowhere" about 10m behind his dinghy.
"I was thinking my dog would only be a mouthful. My heart leaped into my mouth. It could flip me out so easily."
The marine engineer said the leopard seal headed towards a buoy to play with a chain hanging off it.
"Fortunately, it was a memorable moment rather than fatal."
In September, the seal delayed a check for invasive aquatic pests by the National Institute of Water and Atmosphere Auckland marine biosecurity team.
The institute had planned to do a surveillance dive for the Ministry for Primary Industries.
The same month Department of Conservation officers said they had been closely monitoring the seal, and urged people to stay at least 20m away, and not to disturb it or try to feed it.
Leopard seals, typically weighing hundreds of kilograms and measuring several metres as adults, are normally found along the edge of the Antarctic pack ice.
In Winter, young animals move throughout the Southern Ocean and occasionally visit New Zealand.
They've been known to be aggressive when threatened, and can move surprisingly quickly on land and can seriously injure people or animals who approach it.
The large adult female had been hanging around the Hauraki Gulf since July.
